In certain types of communications systems, such as systems employed with wireless telephone systems, information conveyed via analog signalling is received by a receiver in a series of encoded representations of information, generally in the form of a series of "1"'s and "0"'s established via frequency shift keying (FSK) at high frequency. Such transmissions may be subject to reflection such as from buildings and other objects in the transmission path, so there are sometimes radio frequency (RF) signals arriving at a receiver which are time-delayed with respect to other received signals in a manner which may interrupt or distort reception. As a result, the demodulator (or decoder) of the receiver may erroneously convert received signals, thereby becoming unstable and producing interference. Such interference is generally perceived by a user of such a system in the form of popping or clicking sounds or other distracting noises.
It is common in the communications industry to compensate for interference or other noise by providing a feedback circuit with a delay whereby one can estimate the noise component of the received signal, generate a duplicate approximation of the noise component, and subtract that approximate noise component from the original signal to eliminate the noise received in the incoming signal. However, an industry standard published for wireless telephones and similar systems requires that no delays be introduced in such systems; i.e., the system must be what is commonly known as a real-time system. Consequently, a solution for eliminating noise in such a system must likewise be a real-time system.
Real-time noise discrimination and suppression systems have been designed to quickly discriminate and suppress noise in an incoming signal and to effect muting in a manner which does not prevent understanding received speech when the invention mistakenly triggers on true speech. However, when using these noise detection systems, it is possible that high amplitude tones and saturated or clipped speech generated in the transmitter and encoded and sent to the receiver are decoded by the receiver. If the encoding and decoding method is a method such as Adaptive Differential Pulse Code Modulation ("ADPCM"), the decoder may reconstruct the signal in such a way that the decoder output signal is saturated for high amplitude tones and severely clipped for encoded signals that were slightly clipped.
Where the decoder output is saturated or clipped, the noise detection algorithm can falsely trigger the noise suppression algorithm and erroneously mute good tones or speech. Such false triggers can be avoided by reducing the programmed threshold value of the noise detection algorithm. However, reducing the threshold value may reduce the performance of the noise discrimination and detection system in the presence of noise bursts. Such noise bursts may be missed by the system, causing annoying noise signals to be conveyed to the user.